Method for thermally treating a substrate that comprises several layers

ABSTRACT

The aim of the invention is to stop the progression of a lateral oxidation of a layer of a multi-layer substrate at a defined point. To achieve this aim, the invention provides a method for thermally treating a substrate that comprises several layers, especially a semiconductor wafer, according to which a substrate layer that is covered from above and from below is oxidized from the lateral edges thereof to the center in such a manner that a defined center portion is not oxidized. The inventive method comprises the following steps: heating the substrate in a process chamber to a defined treatment temperature; introducing a hydrogen-rich water vapor into the process chamber for a defined period of time; and introducing dry oxygen or an oxygen-rich water vapor into the process chamber once the defined period of time has expired. The hydrogen-rich water vapor can be introduced into the process chamber either before, during and/or after the substrate is heated.

[0001] The present invention relates to the thermal treatment of asubstrate having multiple layers, according to which a substrate layerthat is covered from above and below is oxidized from the side edgesthereof toward the center.

[0002] Such a method is known, for example, during the production of anaperture for surface emitting semiconductor lasers having a verticalresonator, which is also designated as a Vertical Cavity SurfaceEmitting Laser (VCSEL). A VCSEL is a semiconductor laser in which theradiation that is produced is propagated vertically, i.e. in a directionperpendicular to the semiconductor surface and a p-n-junction plane.With conventional lasers, the propagation of the radiation is effectedin a direction parallel to the semiconductor surface and thep-n-junction plane. VCSEL's are used as the preferred light sources forlarge parallel optical communication architectures. To enable a couplingof the VCSEL's with optical fibers, they must emit a Gaussian radiationprofile. For the Gaussian radiation profile, and for a laser activation,a special circular aperture is necessary in the VCSEL, and it must bedefined precisely.

[0003] In the past, this aperture was formed by an oxidation of AlGaAsstructures of the VCSEL's that progressed from the side. For theoxidation, the VCSEL's in a process chamber were brought to a treatmenttemperature and were oxidized with a nitrogen-containing water vapor.After the oxidation, the VCSEL's in the process chamber were cooled inan inert gas atmosphere to a temperature lower than the processtemperature, and they were subsequently removed from the processchamber. In order to increase the throughput, with many VCSEL processes,the substrates are already removed from the process chamber at processtemperatures, with the cooling of the substrates then generally beingeffected in ambient air.

[0004] The oxidation from the side, as is known for example from U.S.Pat. No. 6,014,400, is initiated by the nitrogen-containing water vaportreatment, and an oxidation front is formed that proceeds from theoutside toward the interior. During the cooling-off in inert gasatmospheres, and during removal of the wafers from the process chamber,or during the cooling-off of the substrates having VCSEL structures inambient air, a progression of the oxidation is gradually retarded and isbrought to a stop. This retardation and stopping of the progress of theoxidation front can, however, not be precisely controlled with theabove-mentioned method, so that it is not possible to form a good andpredictable aperture.

[0005] For good optical properties of the VCSEL's, especially for theformation of a desired spatial radiation profile, such as for example aGaussian radiation profile, a precise aperture formation is, however,necessary.

[0006] Proceeding from the above described state of the art, it istherefore an object of the present invention to provide a method withwhich a laterally progressing oxidation of a layer of a multi-layersubstrate is possible such that a defined central portion is notoxidized, i.e. that a progression of the lateral oxidation can bestopped at a specified point.

[0007] Pursuant to the present invention, this object is realized by amethod for the thermal treatment of a substrate, especially asemiconductor wafer, that has multiple layers, according to which asubstrate layer that is covered from above and below is oxidized fromthe side edges thereof toward the center in such a way that a definedcentral portion is not oxidized, whereby the substrate is heated in aprocess chamber to a prescribed treatment temperature, a hydrogen-richwater vapor is introduced into the process chamber for a specifiedperiod of time, and after conclusion of the specified period of time,dry oxygen, or an oxygen-rich water vapor, is introduced into theprocess chamber. As a result of the heating of the substrate to thetreatment temperature, and the introduction of a hydrogen-rich watervapor, it is possible to achieve a controlled, laterally progressingoxidation of a substrate layer that is covered from above and below. Bythe subsequent introduction of dry oxygen or an oxygen-rich water vaporinto the process chamber, it is possible to stop the lateral progressionof the oxidation in a defined manner in order to achieve a defined,non-oxidized central portion. Dry oxygen includes not only pure oxygen(in the form of atomic O and/or molecular O₂ and/or O₃), but also amixture of oxygen and an inert gas such as, for example, nitrogen orargon, whereby the inert gas is distinguished by the fact that it doesnot chemically react with the layers of the substrate. Furthermore, dryoxygen also includes oxygen-containing compounds that contain no water.

[0008] Pursuant to a preferred embodiment of the invention, between theintroduction of the hydrogen-rich water vapor and the introduction ofthe dry oxygen or the oxygen-rich water vapor, a further gas isintroduced into the process chamber in order to displace thehydrogen-rich water vapor out of the process chamber. This prevents anexplosive or detonating gas of hydrogen and oxygen from forming in theprocess chamber, which due to the increased temperature of the processchamber and of the substrate could lead to the danger of an explosion.In this connection, the further gas contains neither hydrogen noroxygen. The further gas is preferably an inert gas in order to avoidundesired reactions with the substrate during this rinsing step.

[0009] For a good and controllable, laterally progressing oxidation ofthe layer that is to be oxidized, the substrate is preferably heated toa treatment temperature of between 300 C and 700 C.

[0010] Pursuant to a particularly preferred embodiment of the invention,the substrate is provided with a semiconductor wafer having an AlGaAsstructure, which is suitable, for example, for the formation of VCSEL's.In this connection, the layer that is to be oxidized is preferably analuminum-containing layer (abbreviated in the following as an aluminumlayer). With the aluminum layer, during the introduction of the dryoxygen or the oxygen-rich water vapor, sapphire is formed thatsuppresses the lateral progression of the oxidation front.

[0011] Advantageously at least one of the plurality of layers forms apreferably round truncated cone, whereby the layer that is to beoxidized is disposed in the region of the truncated cone. This allows awell-defined, round, non-oxidized central portion of the layer that isto be oxidized to be achieved.

[0012] Pursuant to the preferred embodiment of the invention, thenon-oxidized central portion of the layer that is to be oxidized formsan aperture for a Vertical Cavity Surface Emitting Laser (VCSEL). Inthis connection, the aperture is preferably round in order to achieve agood activation of the laser and a Gaussian irradiation profile.

[0013] The temperature of the substrate during the introduction of thehydrogen-rich water vapor and of the dry oxygen or the oxygen-rich watervapor is preferably held at the treatment temperature in order to enablea defined progression of the oxidation front and a controlled halting orstopping of the oxidation front. For an acceleration of the method, thesubstrate is preferably removed from the process chamber directly afterthe step c), which is possible since the oxidation front was halted in acontrolled manner by the introduction of the dry oxygen or theoxygen-rich water vapor. A controlled cooling-off of the substrate inthe process chamber is therefore no longer necessary.

[0014] The introduction of the hydrogen-rich water vapor can be effectedprior to, during and/or after the heating of the substrate to theprescribed treatment temperature, whereby during thermal treatments inrapid heating units (RTP units), the introduction is, however,advantageously effected prior to the heating in order to provideconditions for the laterally progressing oxidation that are defined asprecisely as possible.

[0015] The invention will be explained in greater detail subsequentlywith the aid of a preferred embodiment of the invention with referenceto the drawing. In the drawing:

[0016]FIG. 1 shows a cross-sectional view of an AlGaAs semiconductorstructure for a Vertical Cavity Surface Emitting Laser (VCSEL);

[0017]FIG. 2 shows a schematic cross-sectional illustration through aburner for the provision of a water vapor that contains hydrogen oroxygen;

[0018]FIG. 3 shows a schematic block diagram of a substrate treatmentapparatus for carrying out the inventive method.

[0019]FIG. 1 shows a schematic cross-sectional view through thearrangement of an AlGaAs structure for a Vertical Cavity SurfaceEmitting Laser 1, the light exiting aperture of which is producedpursuant to the method of the present invention.

[0020] The laser has a GaAs substrate on which is applied a firstplurality of n-doped AlGaAs layers. The first plurality of layers isapplied to the GaAs substrate in a region 4, and the layers haveessentially the same peripheral dimensions as does the GaAs substrate 3.Beyond the region 4, the dopings of the layers vary, as indicated inFIG. 1.

[0021] In a further section 6, a second plurality of layers is appliedto the first plurality of layers. In the second section or region 6, theplurality of layers are applied or processed in such a way that theyform a round truncated cone or a mesa, i.e. a layer structure thatprojects beyond the substrate.

[0022] Adjacent to the region or section 4 having the first plurality oflayers first GaAsMQW layers are provided. This is followed by analuminum layer 8 on which is then provided a plurality of p-doped AlGaAslayers. Provided on the upper surface of the truncated cone areelectrical connections or terminals for the activation of the laser, asillustrated by the arrows 10, 11.

[0023] The aluminum layer 8 has an oxidized outer portion 13, which isillustrated as dark-colored, as well as a non-oxidized central portion14 that is illustrated as light-colored and serves as an aperture forthe laser. For a good laser activation, and for a good coupling of thelaser to optical fibers, it is important that the non-oxidized centralportion 14 form a round aperture that is defined as precisely aspossible.

[0024] Such a precisely defined round aperture can be produced by theinventive method, which is described subsequently.

[0025] To form the oxidized outer portion 13, an AlGaAs structure havingthe aforementioned build-up, in which the aluminum layer 8 is present ina continuous non-oxidized state, is loaded into a process chamber of athermal treatment unit. The thermal treatment unit is, for example, arapid heating unit, such as is known from DE-A-199 05 524, whichoriginates from this same applicant, and which to this extent is madethe subject matter of the present invention in order to avoidrepetition.

[0026] The temperature of the AlGaAs structure is subsequently heated toa treatment temperature of between 300 and 700 C, which is advantageousfor a good and defined oxidation of the aluminum layer 8. After theheating of the structure, a hydrogen-rich water vapor is introduced intothe process chamber of the heating unit. The hydrogen-rich water vaporcan be produced, for example, by conducting hydrogen through watervapor. Alternatively, the hydrogen-rich water vapor mixture can also beproduced by the burner, which will be described subsequently withreference to FIGS. 2 and 3.

[0027] As a result of the introduction of the hydrogen-rich water vapor,the following chemical reactions occur within the process chamber:

2AlAs+3H₂O=Al₂O₃+2AsH₃

AlAs+2H₂O=AlO(OH)+AsH₃

AlO(OH)+AlAs+H₂O=Al₂O₃+2AsH₃

AlAs+3H₂O=Al(OH)₃+AsH₃

Al(OH)₃+AlAs=Al₂O₃+AsH₃

2AlAs+6H₂O=Al₂O₃+As₂O₃+6H₂

As₂O₃+6H₂=2AsH₃+3H₂O

[0028] In this connection, there occurs a laterally proceeding oxidationof the aluminum layer, which progresses from the side edges of thelayers 8 toward the inside. The lateral velocity of the oxidation isknown, and after achieving a certain progress of the oxidation, i.e.after termination of a certain period of time, the introduction of thehydrogen-rich water vapor is halted. The process chamber is subsequentlyrinsed with a gas, preferably an inert gas, in order to displace thehydrogen out of the process chamber. Nitrogen and/or noble gases such asargon are preferably used as inert gases.

[0029] After the process chamber is rinsed with the inert gas, dryoxygen or oxygen-rich water vapor is introduced into the processchamber. As a result of the introduction of the dry oxygen or of theoxygen-rich water vapor, the following chemical reaction occurs:

4AlAs+5O₂=2Al₂O₃+4AsO or

2AlAs+3O₂=Al₂O₃+As₂O₃

[0030] In this way, an advancing of the oxidation front by passivationof the oxide layer is achieved. Thus, the advancing of the oxidationfront, and the formation of the non-oxidized aperture 14, can beprecisely controlled in order to form a defined edge region of theaperture for the laser.

[0031] Instead of the rinsing step with an inert gas described above, itis also possible, for the rinsing of the process chamber, to introduceanother gas, or for example pure water vapor, into the process chamberin order after the displacement of the hydrogen-rich water vapor tointroduce the dry oxygen or oxygen-rich water vapor. The displacement ofthe hydrogen prior to the introduction of oxygen is necessary in orderto prevent the formation of an oxyhydrogen or explosive gas mixture inthe process chamber.

[0032] The hydrogen-rich water vapor and/or the oxygen-rich water vaporcan be produced with a burner 20, which is schematically illustrated inthe cross-section of FIG. 2. FIG. 3 schematically shows, in a blockdiagram, an entire unit 22 that is suitable for carrying out theabove-described method, and which contains the burner 20 of FIG. 2 aswell as a thermal treatment unit 24.

[0033] The burner 20 and the unit 22 have, for example, a compositionsuch as is described in the not pre-published German Patent ApplicationNo. 101 19 741 that originates with the same applicant and has the title“Method and Apparatus for the Production of Process Gases” dated 23 Apr.2001, and which to this extent is made the subject matter of the presentinvention in order to avoid repetition.

[0034] In summary, the burner 20 has a housing 23 with a combustionchamber 25. The combustion chamber 25 has an inlet 27 that is incommunication with first and second gas inlet lines 28, 30. By means ofthe inlet lines 28, 30, oxygen or hydrogen can respectively beintroduced into the combustion chamber 25 in a controlled manner. In theregion of the inlet 27, a heating ring 32 that surrounds the inlet lines28, 30 is provided in order to heat the gases introduced via the inletlines 28, 30 and to effect a combustion of the thereby resultingexplosive or detonating mixture of hydrogen and oxygen. The combustionis monitored by an appropriate flame sensor 34. During the combustion,water vapor results by the stoichiometric combustion of substituents ofthe resulting oxygen/hydrogen gas mixture. If one of the substituents ispresent in greater than a stoichiometric proportion, there results anoxygen-rich or hydrogen-rich water vapor mixture that can be introducedinto the thermal treatment unit 24 of FIG. 3 for a thermal treatment ofthe VCSEL 1.

[0035] By increasing the oxygen or hydrogen content, one can alternatebetween an oxygen-rich and a hydrogen-rich water vapor mixture, wherebyduring the switching one must take care that no explosive mixtureresults outside of the burner. This can be achieved, for example, inthat, for example after the production of an oxygen-rich water vapormixture, for a certain period of time stoichiometric proportions ofoxygen and hydrogen are introduced into the burner in order to ensure acomplete combustion and to displace the excess oxygen out of the burner.Only subsequent thereto is additional hydrogen introduced into theburner in order to now produce a hydrogen-rich water vapor mixture.

[0036] Alternatively, the burner can, of course, also be rinsed in themeantime with an inert gas that can also be simultaneously used forrinsing the thermal treatment unit 24.

[0037] As can be recognized in FIG. 3, the entire unit 22, which is usedfor the thermal treatment process of the present invention, has acontrol unit 40 that via appropriate valves 42 controls the supply ofvarious gases into the burner 20 or a connecting line between the burner20 and the thermal treatment unit 24. By introducing oxygen or hydrogeninto a connecting line between the burner 20 and the thermal treatmentunit 24, it is possible to precisely set the oxygen or hydrogen contentof the oxygen-rich or hydrogen-rich water vapor mixture.

[0038] The invention was previously described with the aid of preferredembodiments without being limited to the concretely illustratedembodiments. For example, for the production of an oxygen-rich orhydrogen-rich water vapor mixture, it is not necessary to use a burnerpursuant to FIG. 2. Rather, the mixture could also be produced byevaporating water and introducing oxygen or hydrogen into the vapor.Furthermore, the inventive method for treating a substrate havingmultiple layers can also be carried out at any desired process chamberpressure. For example, the method can be carried out in RTP units atnormal pressure (atmospheric pressure), overpressure or underpressure.At a suitable underpressure and chamber design, a possible explosionpressure of a possible explosive gas explosion can be reduced to such anextent that no damage occurs to the chamber or substrate. In this way, apossible switching between hydrogen-rich and oxygen-rich water vapor (orvice versa) is possible without the chamber being rinsed by an inert gasor pure water vapor for avoiding an explosive gas mixture.

1-15. Cancelled
 16. A method of thermally treating a substrate that has multiple layers, wherein a substrate layer that is covered on opposite sides is oxidized from side edges thereof toward a center thereof such that, via the following steps, a defined central portion is not oxidized: heating the substrate, in a process chamber, to a prescribed treatment temperature; introducing a hydrogen-rich water vapor into the process chamber for a specified period of time, wherein such introduction is effected prior to, during and/or after said step of heating the substrate to the prescribed treatment temperature; and introducing into the process chamber, after conclusion of the specified period of time, one of the group consisting of: dry oxygen, namely pure oxygen in the form of at least one of atomic O, molecular O₂ and O₃; a mixture of oxygen and an inert gas that does not chemically react with the layers of the substrate; an oxygen-containing compound that contains no water; and an oxygen-rich water vapor.
 17. A method according to claim 16, which includes the further step, between said introducing steps, of introducing a further gas into the process chamber to displace hydrogen-rich water vapor out of the process chamber.
 18. A method according to claim 17, wherein said further gas contains neither hydrogen nor oxygen.
 19. A method according to claim 17, wherein said further gas is an inert gas.
 20. A method according to claim 16, wherein said substrate is heated to a treatment temperature of between 300 and 700° C.
 21. A method according to claim 16, wherein the substrate comprises a semi-conductor wafer having a Ill-V semiconductor structure.
 22. A method according to claim 21, wherein said structure is an AlGaAs structure or an InAlAs structure.
 23. A method according to claim 21, wherein the layer that is to be oxidized is a layer containing aluminum.
 24. A method according to claim 16, wherein at least some of the multiple layers form a truncated cone.
 25. A method according to claim 24, wherein said truncated cone is round.
 26. A method according to claim 24, wherein the layer that is to be oxidized is disposed in the region of the truncated cone.
 27. A method according to claim 16, wherein the central portion that is not oxidized forms an aperture for a surface-emitting semiconductor having a vertical resonator, which is also designated as Vertical Cavity Surface Emitting Laser.
 28. A method according to claim 27, wherein the aperture is round.
 29. A method according to claim 16, wherein during the introducing steps, the substrate is held at the prescribed treatment temperature.
 30. A method according to claim 16, wherein immediately after the second introducing step, the substrate is removed from the process chamber.
 31. A method according to claim 16, wherein the process chamber is operated at underpressure. 